Chapter 7 Metamorphism and Metamorphic Rocks
Chapter 7 Summary
The topics covered in this chapter can be summarized as follows:
| 7.1 | Controls over Metamorphic Processes | Metamorphism is controlled by five main factors: the composition of the parent rock, the temperature to which the rock is heated, the amount and type of pressure, the volumes and compositions of aqueous fluids that are present, and the amount of time available for metamorphic reactions to take place. |
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| 7.2 | Classification of Metamorphic Rocks | Metamorphic rocks are classified on the basis of texture and mineral composition. Foliation is a key feature of metamorphic rocks formed under directed pressure; foliated metamorphic rocks include slate, phyllite, schist, and gneiss. Metamorphic rocks formed in environments without strong directed pressure include hornfels, marble, and quartzite. |
| 7.3 | Plate Tectonics and Metamorphism | Almost all metamorphism can be explained by plate-tectonic processes. Oceanic crustal rock can be metamorphosed near the spreading ridge where it was formed, but most other regional metamorphism takes place in areas where mountain ranges have formed, which are most common at convergent boundaries. Contact metamorphism takes place around magma bodies in the upper part of the crust, which are also most common above convergent boundaries. |
| 7.4 | Regional Metamorphism | Geologists classify metamorphic rocks based on some key minerals — such as chlorite, garnet, andalusite, and sillimanite — that only form at specific temperatures and pressures. Most regional metamorphism takes place beneath mountain ranges because the crust becomes thickened and rocks are pushed down to great depths. When mountains erode, those metamorphic rocks are uplifted by crustal rebound. |
| 7.5 | Contact Metamorphism and Hydrothermal Processes | Contact metamorphism takes place around magma bodies that have intruded into cool rocks at high levels in the crust. Heat from the magma is transferred to the surrounding country rock, resulting in mineralogical and textural changes. Water from a cooling body of magma, or from convection of groundwater produced by the heat of the pluton, can also lead to metasomatism, hydrothermal alteration, and accumulation of valuable minerals in the surrounding rocks. |
Exercises
- What are the two main agents of metamorphism, and what are their respective roles in producing metamorphic rocks?
- Into what metamorphic rocks will a mudrock be transformed at very low, low, medium, and high metamorphic grades?
- Why doesn’t granite change very much at lower metamorphic grades?
- Describe the main process of foliation development in a metamorphic rock such as schist.
- What process contributes to metamorphism of oceanic crust at a spreading ridge?
- How do variations in the geothermal gradient affect the depth at which different metamorphic rocks form?
- Blueschist metamorphism takes place within subduction zones. What are the particul